(a) Field of the Invention
The present invention relates to a photocatalyst and a method for preparing the photocatalyst and more particularly to a photocatalyst for use in sterilization, deodorization and prevention of contamination, while making use of the photocatalytic activity thereof.
(b) Description of the Prior Art
Titanium oxide and zinc oxide are semiconductor materials and there has thus been reported applications thereof as photocatalysts having photocatalytic activity. Semiconductor oxide particles are excited through absorption of light rays (in general, ultraviolet rays) having an energy greater than the band gap energy thereof, electrons and positive holes thus generated oxidize or reduce substances absorbed on the surface of the photocatalyst particles to thus decompose the substances through the electron-transfer between the electron and holes and the absorbed substances. Such activity of the semiconductor oxides is in general referred to as photocatalytic activity. The semiconductor oxide particles must directly come in contact with substances to be oxidized or reduced (molecules and ions) and must be irradiated with ultraviolet rays in order that the semiconductor oxide may exhibit this photocatalytic activity.
In fact, most of such photocatalysts consisting of semiconductor oxides are used in the form of fine powder, there has accordingly been used a method comprising the steps of dispersing the fine powder in a liquid such as water containing substances to be decomposed (such as molecules and/or ions) and then irradiating the dispersion with ultraviolet rays.
Alternatively, there has also been reported an attempt to perform sterilization, deodorization and prevention of contamination while making use of such a photocatalyst for the purpose of destroying or decomposing bacterial cells or gases present in a gas phase. In this case, however, it is necessary to suspend the fine particles of a semiconductor oxide having a high activity in the gas phase and then irradiate them with light rays, or to fix the semiconductor oxide fine particles on a substrate and then irradiate the particles with light rays. It is not practicable to yield the photocatalytic activity of semiconductor oxide fine particles while maintaining the suspended state of the particles in a gas phase from the viewpoint of difficulty in the recovery of the particles and installation required for carrying out the process.
There has been proposed a method in which semiconductor oxide fine particles are used after sintering or pressing them into a powder compact, but it is difficult to form a thin film-like compact, i.e., to reduce the weight thereof and therefore, the method is not practicable. In addition, a method in which semiconductor oxide fine particles are added to paint and varnish and then formed into a coated film would also be possible as a method for fixing semiconductor oxide fine particles having a photocatalytic activity on the surface of a substrate. However, the fine particles lose the photocatalytic activity since the surface thereof is covered with a resin present in the paint and varnish. On the other hand, there has been proposed a method for preparing a film of titanium oxide semiconductor fine particles which makes use of the sol-gel technique, but the sol-gel method requires the use of a heating step for the preparation of such a semiconductor oxide film. For this reason, the method is limited in the kinds of substrate used for forming a titanium oxide film and the production cost thereof is high. Japanese Un-examined Patent Publication No. Hei 3-8448 discloses a method for forming a titanium oxide film by a thermal spraying technique. However, when a zinc oxide film is formed by the thermal spraying technique, the film comprises particles of zinc oxide having a large particle size and accordingly, the activity of the film is low. Moreover, the formation of films through a sputtering or vapor deposition technique requires the use of a vacuum or a reduced pressure. Therefore, it is impossible to fix photocatalytic semiconductor oxide fine particles onto walls and/or ceilings presently used, by the sol-gel method or the sputtering or vapor-deposition technique. Moreover, there have been known only a small number of organic substances suitably used for preparing a uniform zinc oxide film by the sol-gel method and there has not yet been known any excellent method for fixing fine powder to substrates. For this reason, there has not been reported any specific example of such a photocatalyst film.
Heretofore, research workers of photocatalysts have mainly focused on titanium oxide. Then the inventors of this invention have conducted various studies and as a result, have found that zinc oxide has a photocatalytic activity comparable to that of titanium oxide and can easily be formed into films and that a film of zinc oxide is thus more practical, as a semiconductor oxide photocatalyst, than a titanium oxide film.
Moreover, the foregoing method which comprises adding semiconductor oxide fine particles having photocatalytic activity to paint and varnish and then formed into a film may be used for applying the semiconductor oxide fine particles onto, for instance, walls and/or ceilings of structures presently used or instruments to thus make the most use of the photocatalytic effect of the fine particles. In this case, however, the surface of the fine particles is covered with a resin component of the paint and varnish and accordingly, the particles would lose their photocatalytic activity. Under such circumstances, the semiconductor oxide fine particles having photocatalytic activity may be applied onto, for instance, walls and/or ceilings of structures or instruments by providing, for instance, paper, a film, a sheet or a plate to the surface of which the fine particles are fixed and then adhering it thereto.